Conventionally, there is known a method for producing a polyurethane resin by reacting a curing agent with an isocyanate group-terminated prepolymer which is obtained by reacting a polyoxyalkylene polyol such as polyoxytetramethylene diol and polyoxypropylene polyol, or a high-molecular-weight polyol such as polyester polyol, with an isocyanate compound.
The high-molecular-weight polyol has been heretofore produced by polymerizing an alkyleneoxide such as ethyleneoxide and propyleneoxide, with an active hydrogen-containing initiator in the presence of a catalyst. As the catalyst, many catalysts are known. For example, as the catalyst used for obtaining a polyoxyalkylene polyol having a small degree of total unsaturation, catalysts such as composite metal cyanide complex catalysts (hereinafter, sometimes referred to as a “DMC catalyst” as needed), cesium-based catalysts, metalloporphyrin catalysts and phosphazene catalysts are known. Above all, the DMC catalyst can be used in a small amount because of its high catalytic activity and in turn, is free of the need to remove the catalyst residue after the polymerization reaction and therefore, this is known as a preferred catalyst for obtaining a polyoxyalkylene polyol where the degree of total unsaturation of the final product polyol is small (Patent Document 1). However, when the polyether polyol produced using a composite metal cyanide complex catalyst is used for the manufacture of an isocyanate group-terminated prepolymer without removing the catalyst residue, the obtained prepolymer may suffer from a problem that, for example, storage stability becomes bad due to the presence of the catalyst residue and the viscosity rises during storage, making the handling difficult. To cope with this problem, a method for deactivating or removing the catalyst residue is being studied.
For example, there have been proposed a purification method of deactivating the DMC catalyst residue in the produced polyether polyol by a heat treatment in the presence or absence of water, treating it with an adsorbent or the like, if desired, and then performing filtration (Patent Document 2); a purification method of water-treating the produced polyether polyol containing a DMC catalyst residue, and after dehydration and addition of a treating agent such as metal, removing the deactivation catalyst, the treating agent and the like (Patent Document 3); and a purification method of subjecting the DMC catalyst residue in the produced polyether polyol to alkali decomposition, subsequently to phosphoric acid neutralization and then to removal by an absorbent (Patent Document 4). However, these methods have a problem that the isocyanate group-terminated prepolymer manufactured using the obtained polyether polyol is insufficient in storage stability or requires a very cumbersome purification operation.
In Patent Document 5, it is stated that a polyether polyol containing from 10 to 1,000 ppm by mass of a DMC catalyst residue has storage stability and storage stability is reasonably ensured also in the isocyanate group-terminated prepolymer using the polyether polyol. This document demonstrates that storage stability was maintained for a relatively short period of time under specific conditions. However, storage stability under normal conditions to which the polyol product is exposed is insufficient, and more improvement of storage stability is keenly demanded. Furthermore, in Patent Document 6, it has been proposed to produce an isocyanate group-terminated prepolymer by using a polyether polyol composition in which from 0.5 to 100 ppm by mass of a phosphoric acid compound is added to an unpurified polyether polyol containing from 1 to 30 ppm by mass of a DMC catalyst, and thereby improve storage stability. However, since a small amount of a phosphoric acid compound is used as an additive, there is still a problem that the operation such as control of the amount added is cumbersome.
On the other hand, for the purpose of achieving deodorization by volatilizing a low-molecular-weight odorous component from the polyether-ol manufactured using a composite metal cyanide complex catalyst, a purification method where an alkyleneoxide is polymerized in the presence of a DMC catalyst and the unpurified polyether-ol is further treated with water vapor in the presence of an acid, preferably, immediately after the completion of polymerization, has been proposed (Patent Document 7). However, this proposal is utterly different in the purpose from the present invention and moreover, requires a large-capacity energy and a coagulator for the supply of water vapor over a long period of time.